NAME

perlpacktut - tutorial on "pack" and "unpack"

DESCRIPTION

"pack" and "unpack" are two functions for transforming data
according to a user-defined template, between the guarded
way Perl stores values and some well-defined representation
as might be required in the environment of a Perl program.
Unfortunately, they're also two of the most misunderstood
and most often overlooked functions that Perl provides. This
tutorial will demystify them for you.

The Basic Principle

Most programming languages don't shelter the memory where
variables are stored. In C, for instance, you can take the
address of some variable, and the "sizeof" operator tells
you how many bytes are allocated to the variable. Using the
address and the size, you may access the storage to your
heart's content.
In Perl, you just can't access memory at random, but the
structural and representational conversion provided by
"pack" and "unpack" is an excellent alternative. The "pack"
function converts values to a byte sequence containing
representations according to a given specification, the so-
called "template" argument. "unpack" is the reverse process,
deriving some values from the contents of a string of bytes.
(Be cautioned, however, that not all that has been packed
together can be neatly unpacked - a very common experience
as seasoned travellers are likely to confirm.)
Why, you may ask, would you need a chunk of memory contain-
ing some values in binary representation? One good reason is
input and output accessing some file, a device, or a network
connection, whereby this binary representation is either
forced on you or will give you some benefit in processing.
Another cause is passing data to some system call that is
not available as a Perl function: "syscall" requires you to
provide parameters stored in the way it happens in a C pro-
gram. Even text processing (as shown in the next section)
may be simplified with judicious usage of these two func-
tions.
To see how (un)packing works, we'll start with a simple tem-
plate code where the conversion is in low gear: between the
contents of a byte sequence and a string of hexadecimal
digits. Let's use "unpack", since this is likely to remind
you of a dump program, or some desperate last message unfor-
tunate programs are wont to throw at you before they expire
into the wild blue yonder. Assuming that the variable $mem
holds a sequence of bytes that we'd like to inspect without
assuming anything about its meaning, we can write
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my( $hex ) = unpack( 'H*', $mem );
print "$hex\n";
whereupon we might see something like this, with each pair
of hex digits corresponding to a byte:
41204d414e204120504c414e20412043414e414c2050414e414d41
What was in this chunk of memory? Numbers, characters, or a
mixture of both? Assuming that we're on a computer where
ASCII (or some similar) encoding is used: hexadecimal values
in the range 0x40 - 0x5A indicate an uppercase letter, and
0x20 encodes a space. So we might assume it is a piece of
text, which some are able to read like a tabloid; but others
will have to get hold of an ASCII table and relive that
firstgrader feeling. Not caring too much about which way to
read this, we note that "unpack" with the template code "H"
converts the contents of a sequence of bytes into the cus-
tomary hexadecimal notation. Since "a sequence of" is a
pretty vague indication of quantity, "H" has been defined to
convert just a single hexadecimal digit unless it is fol-
lowed by a repeat count. An asterisk for the repeat count
means to use whatever remains.
The inverse operation - packing byte contents from a string
of hexadecimal digits - is just as easily written. For
instance:
my $s = pack( 'H2' x 10, map { "3$_" } ( 0..9 ) );
print "$s\n";
Since we feed a list of ten 2-digit hexadecimal strings to
"pack", the pack template should contain ten pack codes. If
this is run on a computer with ASCII character coding, it
will print 0123456789.

Packing Text

Let's suppose you've got to read in a data file like this:
Date |Description | Income|Expenditure
01/24/2001 Ahmed's Camel Emporium 1147.99
01/28/2001 Flea spray 24.99
01/29/2001 Camel rides to tourists 235.00
How do we do it? You might think first to use "split"; how-
ever, since "split" collapses blank fields, you'll never
know whether a record was income or expenditure. Oops. Well,
you could always use "substr":
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while (<>) {
my $date = substr($_, 0, 11);
my $desc = substr($_, 12, 27);
my $income = substr($_, 40, 7);
my $expend = substr($_, 52, 7);
...
}
It's not really a barrel of laughs, is it? In fact, it's
worse than it may seem; the eagle-eyed may notice that the
first field should only be 10 characters wide, and the error
has propagated right through the other numbers - which we've
had to count by hand. So it's error-prone as well as horri-
bly unfriendly.
Or maybe we could use regular expressions:
while (<>) {
my($date, $desc, $income, $expend) =
m|(\d\d/\d\d/\d{4}) (.{27}) (.{7})(.*)|;
...
}
Urgh. Well, it's a bit better, but - well, would you want to
maintain that?
Hey, isn't Perl supposed to make this sort of thing easy?
Well, it does, if you use the right tools. "pack" and
"unpack" are designed to help you out when dealing with
fixed-width data like the above. Let's have a look at a
solution with "unpack":
while (<>) {
my($date, $desc, $income, $expend) = unpack("A10xA27xA7A*", $_);
...
}
That looks a bit nicer; but we've got to take apart that
weird template. Where did I pull that out of?
OK, let's have a look at some of our data again; in fact,
we'll include the headers, and a handy ruler so we can keep
track of where we are.
1 2 3 4 5
1234567890123456789012345678901234567890123456789012345678
Date |Description | Income|Expenditure
01/28/2001 Flea spray 24.99
01/29/2001 Camel rides to tourists 235.00
From this, we can see that the date column stretches from
column 1 to column 10 - ten characters wide. The "pack"-ese
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for "character" is "A", and ten of them are "A10". So if we
just wanted to extract the dates, we could say this:
my($date) = unpack("A10", $_);
OK, what's next? Between the date and the description is a
blank column; we want to skip over that. The "x" template
means "skip forward", so we want one of those. Next, we have
another batch of characters, from 12 to 38. That's 27 more
characters, hence "A27". (Don't make the fencepost error -
there are 27 characters between 12 and 38, not 26. Count
'em!)
Now we skip another character and pick up the next 7 charac-
ters:
my($date,$description,$income) = unpack("A10xA27xA7", $_);
Now comes the clever bit. Lines in our ledger which are just
income and not expenditure might end at column 46. Hence, we
don't want to tell our "unpack" pattern that we need to find
another 12 characters; we'll just say "if there's anything
left, take it". As you might guess from regular expressions,
that's what the "*" means: "use everything remaining".
+ Be warned, though, that unlike regular expressions, if
the "unpack" template doesn't match the incoming data,
Perl will scream and die.
Hence, putting it all together:
my($date,$description,$income,$expend) = unpack("A10xA27xA7xA*", $_);
Now, that's our data parsed. I suppose what we might want to
do now is total up our income and expenditure, and add
another line to the end of our ledger - in the same format -
saying how much we've brought in and how much we've spent:
while (<>) {
my($date, $desc, $income, $expend) = unpack("A10xA27xA7xA*", $_);
$tot_income += $income;
$tot_expend += $expend;
}
$tot_income = sprintf("%.2f", $tot_income); # Get them into
$tot_expend = sprintf("%.2f", $tot_expend); # "financial" format
$date = POSIX::strftime("%m/%d/%Y", localtime);
# OK, let's go:
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print pack("A10xA27xA7xA*", $date, "Totals", $tot_income, $tot_expend);
Oh, hmm. That didn't quite work. Let's see what happened:
01/24/2001 Ahmed's Camel Emporium 1147.99
01/28/2001 Flea spray 24.99
01/29/2001 Camel rides to tourists 1235.00
03/23/2001Totals 1235.001172.98
OK, it's a start, but what happened to the spaces? We put
"x", didn't we? Shouldn't it skip forward? Let's look at
what "pack" in perlfunc says:
x A null byte.
Urgh. No wonder. There's a big difference between "a null
byte", character zero, and "a space", character 32. Perl's
put something between the date and the description - but
unfortunately, we can't see it!
What we actually need to do is expand the width of the
fields. The "A" format pads any non-existent characters with
spaces, so we can use the additional spaces to line up our
fields, like this:
print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
(Note that you can put spaces in the template to make it
more readable, but they don't translate to spaces in the
output.) Here's what we got this time:
01/24/2001 Ahmed's Camel Emporium 1147.99
01/28/2001 Flea spray 24.99
01/29/2001 Camel rides to tourists 1235.00
03/23/2001 Totals 1235.00 1172.98
That's a bit better, but we still have that last column
which needs to be moved further over. There's an easy way to
fix this up: unfortunately, we can't get "pack" to right-
justify our fields, but we can get "sprintf" to do it:
$tot_income = sprintf("%.2f", $tot_income);
$tot_expend = sprintf("%12.2f", $tot_expend);
$date = POSIX::strftime("%m/%d/%Y", localtime);
print pack("A11 A28 A8 A*", $date, "Totals", $tot_income, $tot_expend);
This time we get the right answer:
01/28/2001 Flea spray 24.99
01/29/2001 Camel rides to tourists 1235.00
03/23/2001 Totals 1235.00 1172.98
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So that's how we consume and produce fixed-width data. Let's
recap what we've seen of "pack" and "unpack" so far:
+ Use "pack" to go from several pieces of data to one
fixed-width version; use "unpack" to turn a fixed-width-
format string into several pieces of data.
+ The pack format "A" means "any character"; if you're
"pack"ing and you've run out of things to pack, "pack"
will fill the rest up with spaces.
+ "x" means "skip a byte" when "unpack"ing; when "pack"ing,
it means "introduce a null byte" - that's probably not
what you mean if you're dealing with plain text.
+ You can follow the formats with numbers to say how many
characters should be affected by that format: "A12" means
"take 12 characters"; "x6" means "skip 6 bytes" or "char-
acter 0, 6 times".
+ Instead of a number, you can use "*" to mean "consume
everything else left".
Warning: when packing multiple pieces of data, "*" only
means "consume all of the current piece of data". That's
to say
pack("A*A*", $one, $two)
packs all of $one into the first "A*" and then all of
$two into the second. This is a general principle: each
format character corresponds to one piece of data to be
"pack"ed.

Packing Numbers

So much for textual data. Let's get onto the meaty stuff
that "pack" and "unpack" are best at: handling binary for-
mats for numbers. There is, of course, not just one binary
format - life would be too simple - but Perl will do all
the finicky labor for you.
Integers
Packing and unpacking numbers implies conversion to and from
some specific binary representation. Leaving floating point
numbers aside for the moment, the salient properties of any
such representation are:
+ the number of bytes used for storing the integer,
+ whether the contents are interpreted as a signed or
unsigned number,
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+ the byte ordering: whether the first byte is the least
or most significant byte (or: little-endian or
big-endian, respectively).
So, for instance, to pack 20302 to a signed 16 bit integer
in your computer's representation you write
my $ps = pack( 's', 20302 );
Again, the result is a string, now containing 2 bytes. If
you print this string (which is, generally, not recommended)
you might see "ON" or "NO" (depending on your system's byte
ordering) - or something entirely different if your computer
doesn't use ASCII character encoding. Unpacking $ps with the
same template returns the original integer value:
my( $s ) = unpack( 's', $ps );
This is true for all numeric template codes. But don't
expect miracles: if the packed value exceeds the allotted
byte capacity, high order bits are silently discarded, and
unpack certainly won't be able to pull them back out of some
magic hat. And, when you pack using a signed template code
such as "s", an excess value may result in the sign bit get-
ting set, and unpacking this will smartly return a negative
value.
16 bits won't get you too far with integers, but there is
"l" and "L" for signed and unsigned 32-bit integers. And if
this is not enough and your system supports 64 bit integers
you can push the limits much closer to infinity with pack
codes "q" and "Q". A notable exception is provided by pack
codes "i" and "I" for signed and unsigned integers of the
"local custom" variety: Such an integer will take up as many
bytes as a local C compiler returns for "sizeof(int)", but
it'll use at least 32 bits.
Each of the integer pack codes "sSlLqQ" results in a fixed
number of bytes, no matter where you execute your program.
This may be useful for some applications, but it does not
provide for a portable way to pass data structures between
Perl and C programs (bound to happen when you call XS exten-
sions or the Perl function "syscall"), or when you read or
write binary files. What you'll need in this case are tem-
plate codes that depend on what your local C compiler com-
piles when you code "short" or "unsigned long", for
instance. These codes and their corresponding byte lengths
are shown in the table below. Since the C standard leaves
much leeway with respect to the relative sizes of these data
types, actual values may vary, and that's why the values are
given as expressions in C and Perl. (If you'd like to use
values from %Config in your program you have to import it
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with "use Config".)
signed unsigned byte length in C byte length in Perl
s! S! sizeof(short) $Config{shortsize}
i! I! sizeof(int) $Config{intsize}
l! L! sizeof(long) $Config{longsize}
q! Q! sizeof(long long) $Config{longlongsize}
The "i!" and "I!" codes aren't different from "i" and "I";
they are tolerated for completeness' sake.
Unpacking a Stack Frame
Requesting a particular byte ordering may be necessary when
you work with binary data coming from some specific archi-
tecture whereas your program could run on a totally dif-
ferent system. As an example, assume you have 24 bytes con-
taining a stack frame as it happens on an Intel 8086:
+---------+ +----+----+ +---------+
TOS: | IP | TOS+4:| FL | FH | FLAGS TOS+14:| SI |
+---------+ +----+----+ +---------+
| CS | | AL | AH | AX | DI |
+---------+ +----+----+ +---------+
| BL | BH | BX | BP |
+----+----+ +---------+
| CL | CH | CX | DS |
+----+----+ +---------+
| DL | DH | DX | ES |
+----+----+ +---------+
First, we note that this time-honored 16-bit CPU uses
little-endian order, and that's why the low order byte is
stored at the lower address. To unpack such a (signed) short
we'll have to use code "v". A repeat count unpacks all 12
shorts:
my( $ip, $cs, $flags, $ax, $bx, $cd, $dx, $si, $di, $bp, $ds, $es ) =
unpack( 'v12', $frame );
Alternatively, we could have used "C" to unpack the indivi-
dually accessible byte registers FL, FH, AL, AH, etc.:
my( $fl, $fh, $al, $ah, $bl, $bh, $cl, $ch, $dl, $dh ) =
unpack( 'C10', substr( $frame, 4, 10 ) );
It would be nice if we could do this in one fell swoop:
unpack a short, back up a little, and then unpack 2 bytes.
Since Perl is nice, it proffers the template code "X" to
back up one byte. Putting this all together, we may now
write:
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my( $ip, $cs,
$flags,$fl,$fh,
$ax,$al,$ah, $bx,$bl,$bh, $cx,$cl,$ch, $dx,$dl,$dh,
$si, $di, $bp, $ds, $es ) =
unpack( 'v2' . ('vXXCC' x 5) . 'v5', $frame );
(The clumsy construction of the template can be avoided -
just read on!)
We've taken some pains to construct the template so that it
matches the contents of our frame buffer. Otherwise we'd
either get undefined values, or "unpack" could not unpack
all. If "pack" runs out of items, it will supply null
strings (which are coerced into zeroes whenever the pack
code says so).
How to Eat an Egg on a Net
The pack code for big-endian (high order byte at the lowest
address) is "n" for 16 bit and "N" for 32 bit integers. You
use these codes if you know that your data comes from a com-
pliant architecture, but, surprisingly enough, you should
also use these pack codes if you exchange binary data,
across the network, with some system that you know next to
nothing about. The simple reason is that this order has been
chosen as the network order, and all standard-fearing pro-
grams ought to follow this convention. (This is, of course,
a stern backing for one of the Lilliputian parties and may
well influence the political development there.) So, if the
protocol expects you to send a message by sending the length
first, followed by just so many bytes, you could write:
my $buf = pack( 'N', length( $msg ) ) . $msg;
or even:
my $buf = pack( 'NA*', length( $msg ), $msg );
and pass $buf to your send routine. Some protocols demand
that the count should include the length of the count
itself: then just add 4 to the data length. (But make sure
to read "Lengths and Widths" before you really code this!)
Floating point Numbers
For packing floating point numbers you have the choice
between the pack codes "f" and "d" which pack into (or
unpack from) single-precision or double-precision represen-
tation as it is provided by your system. (There is no such
thing as a network representation for reals, so if you want
to send your real numbers across computer boundaries, you'd
better stick to ASCII representation, unless you're
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absolutely sure what's on the other end of the line.)

Exotic Templates

Bit Strings
Bits are the atoms in the memory world. Access to individual
bits may have to be used either as a last resort or because
it is the most convenient way to handle your data. Bit
string (un)packing converts between strings containing a
series of 0 and 1 characters and a sequence of bytes each
containing a group of 8 bits. This is almost as simple as it
sounds, except that there are two ways the contents of a
byte may be written as a bit string. Let's have a look at an
annotated byte:
7 6 5 4 3 2 1 0
+-----------------+
| 1 0 0 0 1 1 0 0 |
+-----------------+
MSB LSB
It's egg-eating all over again: Some think that as a bit
string this should be written "10001100" i.e. beginning with
the most significant bit, others insist on "00110001". Well,
Perl isn't biased, so that's why we have two bit string
codes:
$byte = pack( 'B8', '10001100' ); # start with MSB
$byte = pack( 'b8', '00110001' ); # start with LSB
It is not possible to pack or unpack bit fields - just
integral bytes. "pack" always starts at the next byte boun-
dary and "rounds up" to the next multiple of 8 by adding
zero bits as required. (If you do want bit fields, there is
"vec" in perlfunc. Or you could implement bit field handling
at the character string level, using split, substr, and con-
catenation on unpacked bit strings.)
To illustrate unpacking for bit strings, we'll decompose a
simple status register (a "-" stands for a "reserved" bit):
+-----------------+-----------------+
| S Z - A - P - C | - - - - O D I T |
+-----------------+-----------------+
MSB LSB MSB LSB
Converting these two bytes to a string can be done with the
unpack template 'b16'. To obtain the individual bit values
from the bit string we use "split" with the "empty" separa-
tor pattern which dissects into individual characters. Bit
values from the "reserved" positions are simply assigned to
"undef", a convenient notation for "I don't care where this
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goes".
($carry, undef, $parity, undef, $auxcarry, undef, $zero, $sign,
$trace, $interrupt, $direction, $overflow) =
split( //, unpack( 'b16', $status ) );
We could have used an unpack template 'b12' just as well,
since the last 4 bits can be ignored anyway.
Uuencoding
Another odd-man-out in the template alphabet is "u", which
packs an "uuencoded string". ("uu" is short for
Unix-to-Unix.) Chances are that you won't ever need this
encoding technique which was invented to overcome the
shortcomings of old-fashioned transmission mediums that do
not support other than simple ASCII data. The essential
recipe is simple: Take three bytes, or 24 bits. Split them
into 4 six-packs, adding a space (0x20) to each. Repeat
until all of the data is blended. Fold groups of 4 bytes
into lines no longer than 60 and garnish them in front with
the original byte count (incremented by 0x20) and a "\n" at
the end. - The "pack" chef will prepare this for you, a la
minute, when you select pack code "u" on the menu:
my $uubuf = pack( 'u', $bindat );
A repeat count after "u" sets the number of bytes to put
into an uuencoded line, which is the maximum of 45 by
default, but could be set to some (smaller) integer multiple
of three. "unpack" simply ignores the repeat count.
Doing Sums
An even stranger template code is "%"<number>. First,
because it's used as a prefix to some other template code.
Second, because it cannot be used in "pack" at all, and
third, in "unpack", doesn't return the data as defined by
the template code it precedes. Instead it'll give you an
integer of number bits that is computed from the data value
by doing sums. For numeric unpack codes, no big feat is
achieved:
my $buf = pack( 'iii', 100, 20, 3 );
print unpack( '%32i3', $buf ), "\n"; # prints 123
For string values, "%" returns the sum of the byte values
saving you the trouble of a sum loop with "substr" and
"ord":
print unpack( '%32A*', "\x01\x10" ), "\n"; # prints 17
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Although the "%" code is documented as returning a "check-
sum": don't put your trust in such values! Even when applied
to a small number of bytes, they won't guarantee a notice-
able Hamming distance.
In connection with "b" or "B", "%" simply adds bits, and
this can be put to good use to count set bits efficiently:
my $bitcount = unpack( '%32b*', $mask );
And an even parity bit can be determined like this:
my $evenparity = unpack( '%1b*', $mask );
Unicode
Unicode is a character set that can represent most charac-
ters in most of the world's languages, providing room for
over one million different characters. Unicode 3.1 specifies
94,140 characters: The Basic Latin characters are assigned
to the numbers 0 - 127. The Latin-1 Supplement with charac-
ters that are used in several European languages is in the
next range, up to 255. After some more Latin extensions we
find the character sets from languages using non-Roman
alphabets, interspersed with a variety of symbol sets such
as currency symbols, Zapf Dingbats or Braille. (You might
want to visit www.unicode.org for a look at some of them -
my personal favourites are Telugu and Kannada.)
The Unicode character sets associates characters with
integers. Encoding these numbers in an equal number of bytes
would more than double the requirements for storing texts
written in Latin alphabets. The UTF-8 encoding avoids this
by storing the most common (from a western point of view)
characters in a single byte while encoding the rarer ones in
three or more bytes.
So what has this got to do with "pack"? Well, if you want to
convert between a Unicode number and its UTF-8 representa-
tion you can do so by using template code "U". As an exam-
ple, let's produce the UTF-8 representation of the Euro
currency symbol (code number 0x20AC):
$UTF8{Euro} = pack( 'U', 0x20AC );
Inspecting $UTF8{Euro} shows that it contains 3 bytes:
"\xe2\x82\xac". The round trip can be completed with
"unpack":
$Unicode{Euro} = unpack( 'U', $UTF8{Euro} );
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Usually you'll want to pack or unpack UTF-8 strings:
# pack and unpack the Hebrew alphabet
my $alefbet = pack( 'U*', 0x05d0..0x05ea );
my @hebrew = unpack( 'U*', $utf );
Another Portable Binary Encoding
The pack code "w" has been added to support a portable
binary data encoding scheme that goes way beyond simple
integers. (Details can be found at Casbah.org, the Scarab
project.) A BER (Binary Encoded Representation) compressed
unsigned integer stores base 128 digits, most significant
digit first, with as few digits as possible. Bit eight (the
high bit) is set on each byte except the last. There is no
size limit to BER encoding, but Perl won't go to extremes.
my $berbuf = pack( 'w*', 1, 128, 128+1, 128*128+127 );
A hex dump of $berbuf, with spaces inserted at the right
places, shows 01 8100 8101 81807F. Since the last byte is
always less than 128, "unpack" knows where to stop.

Template Grouping

Prior to Perl 5.8, repetitions of templates had to be made
by "x"-multiplication of template strings. Now there is a
better way as we may use the pack codes "(" and ")" combined
with a repeat count. The "unpack" template from the Stack
Frame example can simply be written like this:
unpack( 'v2 (vXXCC)5 v5', $frame )
Let's explore this feature a little more. We'll begin with
the equivalent of
join( '', map( substr( $_, 0, 1 ), @str ) )
which returns a string consisting of the first character
from each string. Using pack, we can write
pack( '(A)'.@str, @str )
or, because a repeat count "*" means "repeat as often as
required", simply
pack( '(A)*', @str )
(Note that the template "A*" would only have packed $str[0]
in full length.)
To pack dates stored as triplets ( day, month, year ) in an
array @dates into a sequence of byte, byte, short integer we
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can write
$pd = pack( '(CCS)*', map( @$_, @dates ) );
To swap pairs of characters in a string (with even length)
one could use several techniques. First, let's use "x" and
"X" to skip forward and back:
$s = pack( '(A)*', unpack( '(xAXXAx)*', $s ) );
We can also use "@" to jump to an offset, with 0 being the
position where we were when the last "(" was encountered:
$s = pack( '(A)*', unpack( '(@1A @0A @2)*', $s ) );
Finally, there is also an entirely different approach by
unpacking big endian shorts and packing them in the reverse
byte order:
$s = pack( '(v)*', unpack( '(n)*', $s );

Lengths and Widths

String Lengths
In the previous section we've seen a network message that
was constructed by prefixing the binary message length to
the actual message. You'll find that packing a length fol-
lowed by so many bytes of data is a frequently used recipe
since appending a null byte won't work if a null byte may be
part of the data. Here is an example where both techniques
are used: after two null terminated strings with source and
destination address, a Short Message (to a mobile phone) is
sent after a length byte:
my $msg = pack( 'Z*Z*CA*', $src, $dst, length( $sm ), $sm );
Unpacking this message can be done with the same template:
( $src, $dst, $len, $sm ) = unpack( 'Z*Z*CA*', $msg );
There's a subtle trap lurking in the offing: Adding another
field after the Short Message (in variable $sm) is all right
when packing, but this cannot be unpacked naively:
# pack a message
my $msg = pack( 'Z*Z*CA*C', $src, $dst, length( $sm ), $sm, $prio );
# unpack fails - $prio remains undefined!
( $src, $dst, $len, $sm, $prio ) = unpack( 'Z*Z*CA*C', $msg );
The pack code "A*" gobbles up all remaining bytes, and $prio
remains undefined! Before we let disappointment dampen the
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morale: Perl's got the trump card to make this trick too,
just a little further up the sleeve. Watch this:
# pack a message: ASCIIZ, ASCIIZ, length/string, byte
my $msg = pack( 'Z* Z* C/A* C', $src, $dst, $sm, $prio );
# unpack
( $src, $dst, $sm, $prio ) = unpack( 'Z* Z* C/A* C', $msg );
Combining two pack codes with a slash ("/") associates them
with a single value from the argument list. In "pack", the
length of the argument is taken and packed according to the
first code while the argument itself is added after being
converted with the template code after the slash. This saves
us the trouble of inserting the "length" call, but it is in
"unpack" where we really score: The value of the length byte
marks the end of the string to be taken from the buffer.
Since this combination doesn't make sense except when the
second pack code isn't "a*", "A*" or "Z*", Perl won't let
you.
The pack code preceding "/" may be anything that's fit to
represent a number: All the numeric binary pack codes, and
even text codes such as "A4" or "Z*":
# pack/unpack a string preceded by its length in ASCII
my $buf = pack( 'A4/A*', "Humpty-Dumpty" );
# unpack $buf: '13 Humpty-Dumpty'
my $txt = unpack( 'A4/A*', $buf );
"/" is not implemented in Perls before 5.6, so if your code
is required to work on older Perls you'll need to "unpack(
'Z* Z* C')" to get the length, then use it to make a new
unpack string. For example
# pack a message: ASCIIZ, ASCIIZ, length, string, byte (5.005 compatible)
my $msg = pack( 'Z* Z* C A* C', $src, $dst, length $sm, $sm, $prio );
# unpack
( undef, undef, $len) = unpack( 'Z* Z* C', $msg );
($src, $dst, $sm, $prio) = unpack ( "Z* Z* x A$len C", $msg );
But that second "unpack" is rushing ahead. It isn't using a
simple literal string for the template. So maybe we should
introduce...
Dynamic Templates
So far, we've seen literals used as templates. If the list
of pack items doesn't have fixed length, an expression con-
structing the template is required (whenever, for some rea-
son, "()*" cannot be used). Here's an example: To store
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named string values in a way that can be conveniently parsed
by a C program, we create a sequence of names and null ter-
minated ASCII strings, with "=" between the name and the
value, followed by an additional delimiting null byte.
Here's how:
my $env = pack( '(A*A*Z*)' . keys( %Env ) . 'C',
map( { ( $_, '=', $Env{$_} ) } keys( %Env ) ), 0 );
Let's examine the cogs of this byte mill, one by one.
There's the "map" call, creating the items we intend to
stuff into the $env buffer: to each key (in $_) it adds the
"=" separator and the hash entry value. Each triplet is
packed with the template code sequence "A*A*Z*" that is
repeated according to the number of keys. (Yes, that's what
the "keys" function returns in scalar context.) To get the
very last null byte, we add a 0 at the end of the "pack"
list, to be packed with "C". (Attentive readers may have
noticed that we could have omitted the 0.)
For the reverse operation, we'll have to determine the
number of items in the buffer before we can let "unpack" rip
it apart:
my $n = $env =~ tr/\0// - 1;
my %env = map( split( /=/, $_ ), unpack( "(Z*)$n", $env ) );
The "tr" counts the null bytes. The "unpack" call returns a
list of name-value pairs each of which is taken apart in the
"map" block.
Counting Repetitions
Rather than storing a sentinel at the end of a data item (or
a list of items), we could precede the data with a count.
Again, we pack keys and values of a hash, preceding each
with an unsigned short length count, and up front we store
the number of pairs:
my $env = pack( 'S(S/A* S/A*)*', scalar keys( %Env ), %Env );
This simplifies the reverse operation as the number of
repetitions can be unpacked with the "/" code:
my %env = unpack( 'S/(S/A* S/A*)', $env );
Note that this is one of the rare cases where you cannot use
the same template for "pack" and "unpack" because "pack"
can't determine a repeat count for a "()"-group.

Packing and Unpacking C Structures

In previous sections we have seen how to pack numbers and
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character strings. If it were not for a couple of snags we
could conclude this section right away with the terse remark
that C structures don't contain anything else, and therefore
you already know all there is to it. Sorry, no: read on,
please.
The Alignment Pit
In the consideration of speed against memory requirements
the balance has been tilted in favor of faster execution.
This has influenced the way C compilers allocate memory for
structures: On architectures where a 16-bit or 32-bit
operand can be moved faster between places in memory, or to
or from a CPU register, if it is aligned at an even or
multiple-of-four or even at a multiple-of eight address, a C
compiler will give you this speed benefit by stuffing extra
bytes into structures. If you don't cross the C shoreline
this is not likely to cause you any grief (although you
should care when you design large data structures, or you
want your code to be portable between architectures (you do
want that, don't you?)).
To see how this affects "pack" and "unpack", we'll compare
these two C structures:
typedef struct {
char c1;
short s;
char c2;
long l;
} gappy_t;
typedef struct {
long l;
short s;
char c1;
char c2;
} dense_t;
Typically, a C compiler allocates 12 bytes to a "gappy_t"
variable, but requires only 8 bytes for a "dense_t". After
investigating this further, we can draw memory maps, showing
where the extra 4 bytes are hidden:
0 +4 +8 +12
+--+--+--+--+--+--+--+--+--+--+--+--+
|c1|xx| s |c2|xx|xx|xx| l | xx = fill byte
+--+--+--+--+--+--+--+--+--+--+--+--+
gappy_t
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0 +4 +8
+--+--+--+--+--+--+--+--+
| l | h |c1|c2|
+--+--+--+--+--+--+--+--+
dense_t
And that's where the first quirk strikes: "pack" and
"unpack" templates have to be stuffed with "x" codes to get
those extra fill bytes.
The natural question: "Why can't Perl compensate for the
gaps?" warrants an answer. One good reason is that C com-
pilers might provide (non-ANSI) extensions permitting all
sorts of fancy control over the way structures are aligned,
even at the level of an individual structure field. And, if
this were not enough, there is an insidious thing called
"union" where the amount of fill bytes cannot be derived
from the alignment of the next item alone.
OK, so let's bite the bullet. Here's one way to get the
alignment right by inserting template codes "x", which don't
take a corresponding item from the list:
my $gappy = pack( 'cxs cxxx l!', $c1, $s, $c2, $l );
Note the "!" after "l": We want to make sure that we pack a
long integer as it is compiled by our C compiler. And even
now, it will only work for the platforms where the compiler
aligns things as above. And somebody somewhere has a plat-
form where it doesn't. [Probably a Cray, where "short"s,
"int"s and "long"s are all 8 bytes. :-)]
Counting bytes and watching alignments in lengthy structures
is bound to be a drag. Isn't there a way we can create the
template with a simple program? Here's a C program that does
the trick:
#include <stdio.h>
#include <stddef.h>
typedef struct {
char fc1;
short fs;
char fc2;
long fl;
} gappy_t;
#define Pt(struct,field,tchar) \
printf( "@%d%s ", offsetof(struct,field), # tchar );
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int main() {
Pt( gappy_t, fc1, c );
Pt( gappy_t, fs, s! );
Pt( gappy_t, fc2, c );
Pt( gappy_t, fl, l! );
printf( "\n" );
}
The output line can be used as a template in a "pack" or
"unpack" call:
my $gappy = pack( '@0c @2s! @4c @8l!', $c1, $s, $c2, $l );
Gee, yet another template code - as if we hadn't plenty. But
"@" saves our day by enabling us to specify the offset from
the beginning of the pack buffer to the next item: This is
just the value the "offsetof" macro (defined in
"<stddef.h>") returns when given a "struct" type and one of
its field names ("member-designator" in C standardese).
Neither using offsets nor adding "x"'s to bridge the gaps is
satisfactory. (Just imagine what happens if the structure
changes.) What we really need is a way of saying "skip as
many bytes as required to the next multiple of N". In fluent
Templatese, you say this with "x!N" where N is replaced by
the appropriate value. Here's the next version of our struct
packaging:
my $gappy = pack( 'c x!2 s c x!4 l!', $c1, $s, $c2, $l );
That's certainly better, but we still have to know how long
all the integers are, and portability is far away. Rather
than 2, for instance, we want to say "however long a short
is". But this can be done by enclosing the appropriate pack
code in brackets: "[s]". So, here's the very best we can do:
my $gappy = pack( 'c x![s] s c x![l!] l!', $c1, $s, $c2, $l );
Alignment, Take 2
I'm afraid that we're not quite through with the alignment
catch yet. The hydra raises another ugly head when you pack
arrays of structures:
typedef struct {
short count;
char glyph;
} cell_t;
typedef cell_t buffer_t[BUFLEN];
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Where's the catch? Padding is neither required before the
first field "count", nor between this and the next field
"glyph", so why can't we simply pack like this:
# something goes wrong here:
pack( 's!a' x @buffer,
map{ ( $_->{count}, $_->{glyph} ) } @buffer );
This packs "3*@buffer" bytes, but it turns out that the size
of "buffer_t" is four times "BUFLEN"! The moral of the story
is that the required alignment of a structure or array is
propagated to the next higher level where we have to con-
sider padding at the end of each component as well. Thus the
correct template is:
pack( 's!ax' x @buffer,
map{ ( $_->{count}, $_->{glyph} ) } @buffer );
Alignment, Take 3
And even if you take all the above into account, ANSI still
lets this:
typedef struct {
char foo[2];
} foo_t;
vary in size. The alignment constraint of the structure can
be greater than any of its elements. [And if you think that
this doesn't affect anything common, dismember the next
cellphone that you see. Many have ARM cores, and the ARM
structure rules make "sizeof (foo_t)" == 4]
Pointers for How to Use Them
The title of this section indicates the second problem you
may run into sooner or later when you pack C structures. If
the function you intend to call expects a, say, "void *"
value, you cannot simply take a reference to a Perl vari-
able. (Although that value certainly is a memory address,
it's not the address where the variable's contents are
stored.)
Template code "P" promises to pack a "pointer to a fixed
length string". Isn't this what we want? Let's try:
# allocate some storage and pack a pointer to it
my $memory = "\x00" x $size;
my $memptr = pack( 'P', $memory );
But wait: doesn't "pack" just return a sequence of bytes?
How can we pass this string of bytes to some C code
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expecting a pointer which is, after all, nothing but a
number? The answer is simple: We have to obtain the numeric
address from the bytes returned by "pack".
my $ptr = unpack( 'L!', $memptr );
Obviously this assumes that it is possible to typecast a
pointer to an unsigned long and vice versa, which frequently
works but should not be taken as a universal law. - Now that
we have this pointer the next question is: How can we put it
to good use? We need a call to some C function where a
pointer is expected. The read(2) system call comes to mind:
ssize_t read(int fd, void *buf, size_t count);
After reading perlfunc explaining how to use "syscall" we
can write this Perl function copying a file to standard out-
put:
require 'syscall.ph';
sub cat($){
my $path = shift();
my $size = -s $path;
my $memory = "\x00" x $size; # allocate some memory
my $ptr = unpack( 'L', pack( 'P', $memory ) );
open( F, $path ) || die( "$path: cannot open ($!)\n" );
my $fd = fileno(F);
my $res = syscall( &SYS_read, fileno(F), $ptr, $size );
print $memory;
close( F );
}
This is neither a specimen of simplicity nor a paragon of
portability but it illustrates the point: We are able to
sneak behind the scenes and access Perl's otherwise well-
guarded memory! (Important note: Perl's "syscall" does not
require you to construct pointers in this roundabout way.
You simply pass a string variable, and Perl forwards the
address.)
How does "unpack" with "P" work? Imagine some pointer in the
buffer about to be unpacked: If it isn't the null pointer
(which will smartly produce the "undef" value) we have a
start address - but then what? Perl has no way of knowing
how long this "fixed length string" is, so it's up to you to
specify the actual size as an explicit length after "P".
my $mem = "abcdefghijklmn";
print unpack( 'P5', pack( 'P', $mem ) ); # prints "abcde"
As a consequence, "pack" ignores any number or "*" after
"P".
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Now that we have seen "P" at work, we might as well give "p"
a whirl. Why do we need a second template code for packing
pointers at all? The answer lies behind the simple fact that
an "unpack" with "p" promises a null-terminated string
starting at the address taken from the buffer, and that
implies a length for the data item to be returned:
my $buf = pack( 'p', "abc\x00efhijklmn" );
print unpack( 'p', $buf ); # prints "abc"
Albeit this is apt to be confusing: As a consequence of the
length being implied by the string's length, a number after
pack code "p" is a repeat count, not a length as after "P".
Using "pack(..., $x)" with "P" or "p" to get the address
where $x is actually stored must be used with circumspec-
tion. Perl's internal machinery considers the relation
between a variable and that address as its very own private
matter and doesn't really care that we have obtained a copy.
Therefore:
+ Do not use "pack" with "p" or "P" to obtain the address
of variable that's bound to go out of scope (and thereby
freeing its memory) before you are done with using the
memory at that address.
+ Be very careful with Perl operations that change the
value of the variable. Appending something to the vari-
able, for instance, might require reallocation of its
storage, leaving you with a pointer into no-man's land.
+ Don't think that you can get the address of a Perl vari-
able when it is stored as an integer or double number!
"pack('P', $x)" will force the variable's internal
representation to string, just as if you had written
something like "$x .= ''".
It's safe, however, to P- or p-pack a string literal,
because Perl simply allocates an anonymous variable.